for letting Akka HTTP manage a connection-pool to one specific host/port endpoint. Recommended when the user can supply a Source<HttpRequest, ?>Source[HttpRequest, _] with requests to run against a single host over multiple pooled connections.

for full control over when HTTP connections are opened/closed and how requests are scheduled across them. Only recommended for particular use cases.

You can interact with different API levels at the same time and, independently of which API level you choose, Akka HTTP will happily handle many thousand concurrent connections to a single or many different hosts.